Swift: Beta 4, 5 changes and highlighting improvements

Hi! I picked up where @ole left off in #167 and:

- fixed the problem with `private(set)`/`internal(set)` not being highlighted properly
- updated the list of attributes and declaration modifiers for Beta 5
- added coloring of classes/protocols in @objc(Class) attributes and in class, protocol and extension declarations

Note that I don't _really_ know what I'm doing, but after playing around with the visual samples, it looks good to me!
oh! the one thing i might do differently is

``` rb
rule(//) { pop! }
```

so you don't have to give it a dummy token.